Continuously calcined gypsum (stucco), because of processing conditions in its preparation, differs in physical properties from stuccos made by other processes. When gypsum is continuously calcined, a high number of small fractures occur in the calcium sulfate particles. During wet mixing of continuously calcined gypsum with water to form a slurry, very small fractions of the calcium sulfate particles rapidly break from the particle because of the fissuring of the fractured particle. Thus, for example, the mean particle size of the continuously calcined gypsum particles after calcination may be approximately 19 microns, and during wet mixing of that stucco, 1 to 3 micron sized fragments will rapidly flake off the particles. This disintegration is a desirable characteristic in some respects, for it provides a high rate of hydration or "set suddeness", and a high ultimate strength of the set gypsum because of the high surface area of the fragments. This disintegration of the particles, however, also results in undesirable high water demand due to the much higher surface area of the stucco fragments.
In automated gypsum board manufacture, a large portion of the processing time and processing energy is devoted to removing excess water from the wet board. Considerable excess water is required in gypsum board manufacture to properly fluidize the calcined gypsum and obtain proper flow of the gypsum slurry. Thus, calcined gypsum made by continuous calcination may have a dispersed consistency of about 100-150 cc. "Dispersed consistency" for purposes of gypsum board manufacture in accordance with the present invention may be defined as the water volume required to give a standard viscosity or flow when 100 grams of calcined gypsum is dispersed by mechanical mixing in a high speed blender for 7 seconds which is equivalent to that encountered in the board forming line. While the dispersed consistency may be expressed in a particular numerical figure, it will be appreciated that any particular number is variable from one processing line to the next depending on the particular stucco and the amount of flow for that stucco being most desirable for that particular processing line.
A dispersed consistency value of 100-150 cc. indicates a water requirement of about 85-100 parts of water per 100 parts of the calcined gypsum for an acceptable slurry in a modern automated gypsum board plant. The theoretical water required to convert the calcined gypsum (calcium sulfate hemihydrate or stucco) to set gypsum dihydrate is only 18.7% by weight on a pure basis. This leaves about 67to about 82% of the water present in the gypsum slurry to be removed in drying the board. Ordinarily, gypsum board dryers in an automated gypsum board manufacturing line will remove this water, for example, by maintaining the temperature at about 400.degree. F. and requiring a drying time of about 40 minutes. Of course, the time-temperature relationship is variable from one processing plant to another depending upon the particular gypsum source and processing equipment.
In addition, line speed of the board slurry line is affected by the setting time and the set suddeness of the calcined gypsum slurry. Thus, calcined gypsum made by continuous calcination will ordinarily be adjusted to have a temperature rise set of about 8 minutes and a set suddeness rate of about 9.degree. F. per minute.
In my copending U.S. Patent Application Ser. Nos. 777,213 and 788,953 there are described apparatus and processes for continuously treating calcined gypsum so as to lower the water demand and provide a treated gypsum mass which may be continuously fed into the slurry mixer of an automated gypsum board line. The treatment comprises thoroughly blending small amounts of water into the calcined gypsum, resulting in a damp but dry appearing material and allowing it to "heal" before usage in gypsum board production. By "healing" is meant allowing the small amounts of free water to remain on the calcined gypsum particle surface for a short period of time, about 1-10 minutes, and it is believed that this permits the fractures on the particles to fuse so as to resist subsequent disintegration of the particle into numerous micron sized fractions during slurry mixing. The healed stucco is particularly suitable for immediate utilization in gypsum board manufacture; however, if such material is not used right away, the set suddeness of the healed stucco begins to vary erratically. "Set suddeness", for purposes of the present invention, may be defined as the maximum rate of temperature rise of the exothermic stucco temperature-time curve. The development of set suddeness is important in providing adequate ultimate strength of the cast gypsum product.
It has been found that board manufactured by the process of adding a small amount of water to the calcined gypsum to reduce water demand is considerably weaker at the same board core density than if untreated calcined gypsum were used in making the board. The calcined gypsum produced by the continuous process, depending on the gypsum source, may be low in ultimate strength development, and the further reduction in ultimate strength resulting from the above-described processes of water treatment may produce an unacceptable board. Further, the water-treated calcined gypsum of such process should be used in gypsum board production immediately after healing or else dried after healing, for the water-treated calcined gypsum is not storage stable.